[experiments][api] Fix exception declaration in except block

beat/web/experiments/api.py

@@ -470,8 +470,8 @@ class RetrieveUpdateDestroyExperimentView(RetrieveUpdateDestroyContributionView)
 
         try:
             experiment.save()
-        except SyntaxError, e:
-            return BadRequestResponse(e.message)
+        except SyntaxError as e:
+            return BadRequestResponse(str(e))
 
         # Create the in-memory representation of the toolchain
         core_obj = experiment.core()